Keizer was appointed the new head coach of Sporting earlier this month and is set to oversee a clearout in January in order to raise funds for new players.

According to O Jogo, Castaignos is on the list of players that can exit the club alongside Emiliano Viviano, Jefferson Nascimento, Radosav Petrovic, Josip Mišić and Bruno César.

The 26-year-old Dutch forward joined Sporting in 2016 from Eintracht Frankfurt, but has barely played in Portugal. Last season, Castaignos was loaned to Vitesse Arhem, but did not impress. The former Inter Milan and Twente striker has made four appearances for Sporting this season without scoring.